If a desktop computer get a video card with Dual-Display option. Otherwise if a laptop that is unable to slow both or desktop you dont want to upgrade video card, then you will need a video splitter a box that takes in 1 input and outputs multiple outputs. Lots of computer stores use these for there monitor displays so I system sends its video signal to a box that duplicates the output to liek 8 or more monitors for side by side comparision between models etc.
A Video card with dual-display feature can be purchased for around $100 or so. Not sure on the price of a video splitter box for VGA.